1. Field of the Invention
The present invention relates to optical spectroscopy and more particularly to the optical spectroscopy of a surface of a sample.
2. Description of Related Art
Absorption and transmission spectra of a sample can be obtained using electromagnetic radiation for the purpose of identifying chemical types. Certain spectral regions are of greater use than others. The spectral regions of greatest use are those in which chemical species contained within the sample, absorb radiation at characteristic wavelengths. The absorbed radiation can be detected directly, re-emitted by the sample or cause the sample to fluoresce. Spectroscopic measurements of absorption, emission and fluorescence using infra-red, visible and ultra-violet light are widely used for sample analysis. Unfortunately, many of these techniques are not well suited to applications where the sample has rough surfaces. Radiation is scattered by rough surfaces thereby making measurement of reflection difficult. Techniques requiring transmission through a sample, are difficult or impossible using opaque samples.
One technique which has proved useful for the spectroscopic analysis of rough surfaces is Photo-Acoustic Spectroscopy. With this technique, a monochromatic light source is chopped to create a pulsed light source. The pulsed light is directed to impinge on the surface of the sample. The pulsed light is at a wavelength which is absorbed by the sample, causing a periodic heating of the surface which in turn causes the surface to expand and contract. This movement generates sound waves which propagate through the gas above the surface and are detected by a sensitive microphone. Spectra of the surface are generated by varying the wavelength of the incident radiation. Unfortunately, the detection efficiency of Photo-Acoustic procedures is low with respect to other optical analysis methods, and thus the technique is not easily applied to small samples.
A non-spectroscopic technique which has been demonstrated to be useful for the analysis of surface roughness is Optical Interferometry. In this technique, monochromatic light impinges upon a sample surface resulting in interferograms. The resulting interferograms are then used to generate digital maps of the surface. While this technique detects the physical features of a surface, it is not capable of providing information about the surface's chemical composition. A review of the techniques of optical interferometry of surfaces is described in Optical Interferometry Scientific American July 1991, pages 44-49 by Glen M. Robinson, Ph.D., David M. Perry, and Richard W. Peterson which is hereby incorporated by reference.
Presently there is a need for an analytical system capable of detecting the optical spectra of selected chemical types for samples having rough surfaces, with high sensitivity.